FIELD OF THE INVENTION
The invention relates to a thyristor-switched capacitor bank having a thyristor switch and a capacitor bank.
A solid-state compensator, which is also referred to as a Static Var Compensator (SVC) includes one or more parallel-connected inductive and capacitive paths, which are connected to a high-voltage mains power supply through a dedicated transformer or else through a tertiary winding of a mains power supply transformer. As a result of a rated voltage on the secondary side of the transformer being fixed, the use of a dedicated transformer offers the capability of constructing the equipment optimally in terms of its current and voltage control. A direct connection may also be economical in medium-voltage mains power supplies up to 30 kV.
The total amount of capacitance is provided through permanently connected or switched capacitors (capacitor bank) that are also referred to as a Fixed Capacitor (FC), or thyristor-switched capacitors that are also referred to as a Thyristor Switched Capacitor (TSC). A thyristor switch which includes a plurality of series-connected, reverse-parallel or anti-parallel thyristors is normally used for that application. The capacitor bank must then be provided with a protective inductor, in order to limit an inrush current gradient. The use of mechanically switched capacitors is subject to operational limitations. The capacitor bank must always be discharged through a power switch during switching on (for example through a discharge resistor or a transformer) in order to keep equalization processes during switching-on as small as possible and thus to prevent overloading. In comparison therewith, a thyristor as a switch offers the advantage of permitting the capacitor bank to be connected and disconnected from any charge state and as frequently as desired with the minimum possible equalization process. The controller (intelligence) which is required for that purpose can easily be implemented by using digital technology.
The total amount of inductance is provided through inductor coils. They can either be switched (Thyristor Switched Reactor (TSR)), or else the reactive volt-amperes at the fundamental frequency can be controlled (Thyristor Controlled
Reactor (TCR)) using an appropriate controller. To that end, the entire amount of the reactive volt-amperes emitted to the mains power supply from the solid-state compensator can be adjusted infinitely variably in terms of the capacitive or inductive reactive volt-amperes required at the mains power supply point.
Continuous control of a TCR path is always linked to the production of harmonic currents, which must be kept away from the transmission grid by the use of filters at the TCR connection point. The production of harmonics can be completely prevented only by the inductive path being operated in such a way that it is switched identically to the capacitive path (Thyristor Switched Reactor (TSR)). The installed inductive volt-amperes are then only connected or disconnected in the same way as in the case of a thyristor-switched capacitor bank (Thyristor Switched Capacitor (TSC)).
In principle, the solid-state compensator can carry out various control tasks. When used in transmission grids, the primary task is voltage control. The solid-state compensator can thus also contribute to limiting overvoltages at the operating frequency, can make a contribution to improving the grid stability and can also damp volt-ampere fluctuations between grid sections.
An article entitled "Statische Kompensatoren und ihre Komponenten" Solid-State Compensators and their Components!, printed in the German journal "etz", Volume 112 (1991), Issue 17, pages 926 to 930, discusses circuit types, application and structural criteria for the components used in solid-state compensators using thyristor technology. The solid-state compensators which are implemented and referred to each include a plurality of power-factor correctors, which are connected to a high-voltage mains power supply through the use of a transformer. The selection and combination of the various power-factor correctors depends essentially on the requirements of the mains power supply. The following viewpoints, inter alia, have to be considered in that case: total cost of the compensator, loss assessment, reliability, maintenance costs and the capability of the compensator to be upgraded. For example, the SVC system at Kemps Creek, Australia includes a thyristor-switched inductor (TSR) and two thyristor-switched capacitor banks (TSC). The three phases of each of those power-factor correctors are electrically connected in delta and are of identical construction.
As already mentioned, the capacitor bank of the thyristor-switched capacitor bank (TSC) should always be discharged during switching-on. As a rule, the capacitor bank is disconnected from the AC mains power supply at the current zero crossing, that is to say at the instant when the mains power supply voltage is at a maximum. If the discharging of the capacitor bank through a discharge circuit is a slow process in comparison with the period of the AC voltage, then virtually twice the maximum mains power supply voltage occurs on the thyristor switch after half a cycle. Relatively expensive thyristors having an increased withstand voltage must be used for the thyristor switch, or a plurality of thyristor switches must be connected in series. If incorrect triggering of a thyristor were then to occur at the least favorable point in time, the capacitor bank would be recharged to a maximum of three times the mains power supply voltage amplitude.
In order to ensure that the thyristor switch only need be constructed for the maximum mains power supply voltage itself, which is a major advantage for economic reasons, the capacitor bank must be able to be discharged through a discharge circuit sufficiently quickly, at the most within one half-cycle of the AC voltage. If the AC voltage frequency is 50 Hz, the duration of one half cycle is 10 ms. The capacitor bank normally has a capacitance on the order of magnitude of several 100 .mu.F. In order for it to be possible for such a large capacitor bank to be able to be discharged in 10 ms at all, the discharge circuit must have a low impedance. For example, a purely non-reactive resistor in the discharge circuit must have a value of only a few ohms which, for the capacitor, represents virtually a short circuit with a correspondingly high power loss, that cannot be tolerated when the capacitor bank is connected to the AC mains power supply.
European Patent 0 116 275 B1 discloses a reactive volt-ampere compensator, a discharge circuit having at least one inductive impedance element being connected in parallel with a thyristor-switched capacitor bank, and a first control unit being provided for the thyristor switch. The first control unit produces triggering signals for the thyristor switch from current and voltage measurement signals from an AC mains power supply which is to be corrected. The discharge circuit is permanently closed and the inductive impedance element is variable in such a manner that its value is greater in the operating state when the thyristor switch is closed and is less when the thyristor switch is open. One advantage of that device is that rapid and continuous discharging of the capacitor bank, after it has been disconnected from the AC mains power supply, takes place without any switching elements in the discharge circuit of the capacitor bank. Such switching elements would be susceptible to defects and would be expensive. An iron-cored discharge-circuit inductor is provided as the inductive impedance element. The iron core is at least largely unsaturated at that current which flows through the inductor when the thyristor switch is closed, and is increasingly saturated with greater currents. Its winding impedance is selected in such a way that the discharging of the capacitor bank corresponds to an RC discharge with a priori damping. As a result of the saturation characteristics of its iron core, the discharge-circuit inductor thus acts as a variable impedance element in the discharge circuit, having an impedance which is greater when the capacitor is being connected to the AC mains power supply, that is to say when the thyristor switch is closed, than when the capacitor bank is disconnected from the AC mains power supply with the thyristor switch open. The difference between those two states in that case is so significant that only a small, insignificant current flows in the first-mentioned case during discharge, while a greater current, which discharges the capacitor bank in less than one half-cycle of the AC voltage, can flow in the second case. In addition, the discharge circuit may be permanently closed. There is no need for any interruption in the charging circuit while the capacitor bank is connected to the AC mains power supply. That results in the thyristor voltage being relatively low, and the costs of expensive high-voltage thyristors are thus saved.